This invention relates to a semiconductor laser device in which a plurality of semiconductor lasers are formed on one semiconductor substrate and a manufacturing method therefor.
In recent years, optical disks have been popularized, and the recording formats thereof have had numbers of variations. When optically reading optical disks of different standards, semiconductor lasers of different standards are needed. For example, in order to read two types of optical disks of CD (Compact Disc) and DVD (Digital Versatile Disc), there are needed an infrared laser of an emission wavelength of about 780 nm and a red laser of an emission wavelength of about 650 nm.
In the above case, there has been demanded the appearance of a semiconductor laser device capable emitting laser light rays of two wavelengths in one package for the downsizing and cost reduction of the pickup.
Moreover, there has been demanded the appearance of a semiconductor laser device capable of emitting two laser light rays of two wavelengths in one package or emitting laser light rays of two types for a low output and a high output even at same wavelength for laser light printers and recording-and-reproducing type optical disks other than the optical disks. Furthermore, a laser of two beams of same outputs having same wavelength can also be considered.
In order to satisfy these demands, there has been developed the technology of providing two semiconductor lasers on one semiconductor substrate. However, when forming semiconductor lasers of two different characteristics on a single semiconductor substrate, it is often the case where the device cannot be formed through one-time crystal growth. Accordingly, the present applicant has proposed the method of carrying out crystal growth a plurality of times on a single semiconductor substrate although the method has not been yet known, i.e., is not a prior art. That is, the method has the processes of firstly growing one laser structure on a semiconductor substrate, partially etching the laser structure crystallinically grown at the first time for the exposure of the semiconductor substrate, thereafter crystallinically growing another laser structure superposed on them at the second time and removing the portion of the subsequently formed laser structure on the precedently grown laser structure.
FIGS. 3A, 3B and 3C through 3G show the cross-sections of a semiconductor laser device in which two semiconductor lasers of an AlGaAs based semiconductor laser and an AlGaInP based semiconductor laser are grown on a GaAs substrate. First of all, as shown in FIG. 3A, an AlGaAs based semiconductor laser 9 constructed of an n-type GaAs buffer layer 2, an n-type AlGaAs first cladding layer 3, an AlGaAs first guide layer 4, a multiple quantum well active layer 5, an AlGaAs second guide layer 6, a p-type AlGaAs second cladding layer 7 and a p-type GaAs contact layer (Zn-doped) 8 is grown on an n-type GaAs substrate 1. Then, as shown in the right-hand side portion in FIG. 3A, a partial region of the AlGaAs based semiconductor laser 9 is removed by etching until the n-type GaAs substrate 1 is exposed.
Subsequently, as shown in FIG. 3B, an AlGaInP based semiconductor laser 18 constructed of an n-type GaAs buffer layer 11, an n-type AlGaInP first cladding layer 12, an AlGaInP first guide layer 13, a multiple quantum well active layer 14, an AlGaInP second guide layer 15, a p-type AlGaInP second cladding layer 16 and a p-type GaAs contact layer 17 is grown all over the surface.
Next, as shown in FIG. 3C, a region of the subsequently formed AlGaInP based semiconductor laser 18 superposed on the precedently formed AlGaAs based semiconductor laser 9 is removed by etching. Further, a boundary portion located between the AlGaAs based semiconductor laser 9 and the AlGaInP based semiconductor laser 18 on the n-type GaAs substrate 1 is removed until the n-type GaAs substrate 1 is exposed, forming a semiconductor laser device in which the AlGaAs based semiconductor laser 9 and the AlGaInP based semiconductor laser 18 are juxtaposed on the n-type GaAs substrate 1.
Subsequently, as shown in FIG. 3D, the p-type GaAs contact layer 8 and the p-type AlGaAs second cladding layer 7 of the AlGaAs based semiconductor laser 9 are removed wholly and partway, respectively, by etching so that only the center portion is left by a prescribed width, forming a stripe-shaped ridge portion 10 in the center portion. At the same time, the p-type GaAs contact layer 17 and the p-type AlGaInP second cladding layer 16 of the AlGaInP based semiconductor laser 18 are removed wholly and partway, respectively, by etching, forming a stripe-shaped ridge portion 20 in a center portion.
Subsequently, as shown in FIG. 3E, an n-type GaAs current constriction layer 21 is grown all over the AlGaAs based semiconductor laser 9 and the AlGaInP based semiconductor laser 18. Subsequently, as shown in FIG. 3F, an unnecessary portion of the n-type GaAs current constriction layer 21 located on upper portions of the ridge portions 10 and 20 and an element isolating portion 22 are removed by etching, so that currents flow only in the ridge portions 10 and 20.
Subsequently, as shown in FIG. 3G, a p-type AuZn/Au electrode 23 is formed on the whole surface of the AlGaAs based semiconductor laser 9. At the same time, a p-type AuZn/Au electrode 24 is formed on the whole surface of the AlGaInP based semiconductor laser 18. Further, an n-type AuGe/Ni electrode 25 is formed on the whole back surface of the n-type GaAs substrate 1.
Thus, as shown in FIG. 3G, there is formed a semiconductor laser device, in which the two semiconductor lasers of the AlGaAs based semiconductor laser 9 and the AlGaInP based semiconductor laser 18 are provided on one n-type GaAs substrate 1.
However, the aforementioned semiconductor laser device manufacturing method for carrying out the crystal growth a plurality of times on the single semiconductor substrate has the following problems. That is, as shown in FIG. 3E, the n-type GaAs current constriction layer 21 is grown all over the AlGaAs based semiconductor laser 9 and the AlGaInP based semiconductor laser 18, and therefore, the n-type GaAs current constriction layer 21 is also formed on the ridge portions 10 and 20. Therefore, it is required to remove the n-type GaAs current constriction layer 21 formed on the ridge portions 10 and 20 so that currents flow in the ridge portions 20 and 21. The removal in the above-mentioned case is carried out by protecting the portions other than the upper portions of the exposed ridge portions 10 and 20 and the element isolating portion 22 with a resist and by etching only exposed upper portions on the ridge portions 10 and 20 and element isolating portion 22.
However, as shown in FIGS. 4A, 4B and 4C, the unnecessary n-type GaAs current constriction layer 21, which is formed on the ridge portions 10 and 20 (represented by the ridge portion 10 located on the AlGaAs based semiconductor laser 9 side in the figure), grows in a trapezoidal shape that has the width of the upper surfaces of the ridge portions 10 and 20 roughly as the base, and the trapezoid comes to have an increasing height as the thickness of the n-type GaAs current constriction layer 21 increases and comes to have a shape close to a triangle.
Therefore, when this unnecessary n-type GaAs current constriction layer 21 having the shape close to a triangle is removed by etching as shown in FIGS. 5A and 5B (represented by the ridge portion 10), since the etching progresses roughly uniformly, the etching is required to be effected more deeply in order to completely expose the upper surfaces of the ridge portions 10 and 20. Therefore, in the portion of the n-type GaAs current constriction layer 21 which extends along the edge portions of the upper surface of the ridge portion 10 and which is thin as shown in FIG. 5A, the etching reaches down to the p-type second cladding layer 7 as shown in FIG. 5B, and the p-type second cladding layer 7 of the ridge portion 10 is disadvantageously exposed. Although not shown, the same thing can be said for the p-type second cladding layer 16 of the ridge portion 20.
Then, the neighborhood of the active layer for emitting laser light is exposed, and the confinement of the laser light becomes unstable. Moreover, the laser characteristics also deteriorate. It is to be noted that the reference numeral 26 denotes a resist in FIGS. 5A and 5B.
Therefore, when etching the unnecessary n-type GaAs current constriction layer 21, it is required to expose neither of the p-type second cladding layers 7 and 16, and therefore, the etching requires very advanced controllability. Furthermore, when a plurality of semiconductor lasers are integrally formed on a single semiconductor substrate, it is required to concurrently control the etching of a plurality of ridge portions, and this makes it more difficult to etch the unnecessary n-type GaAs current constriction layer 21.
As a countermeasure against the aforementioned problem, there can be considered a method for reducing the film thickness of the n-type GaAs current constriction layer 21 to be grown in order to suppress low the height of the trapezoid of the n-type GaAs current constriction layer 21 on the ridge portions 10 and 20 as shown in FIG. 6A (only the ridge portion 10 is shown, and the same thing can be said for the ridge portion 20). However, a current leaks out of the ridge portions 10 and 20 as the film thickness of the n-type GaAs current constriction layer 21 is reduced, disadvantageously loosing the current confinement effect. In the above case, the leak current is increased to deteriorate the reliability, further causing the occurrence of no laser oscillation obtained.
Moreover, as shown in FIG. 6B, there may be the case where eaves-like protrusions are generated on the p-type GaAs contact layer 8 due to a difference between the compositions of the p-type GaAs contact layer 8 and the p-type second cladding layer 7 during the etching in forming the ridge portion 10 (see FIG. 3D). If the n-type GaAs current constriction layer 21 is thinly formed by the MBE (Molecular Beam Epitaxy) method in this state, then the n-type GaAs current constriction layer 21 is not grown on the back surface side of the eaves of the p-type GaAs contact layer 8 as shown in FIG. 6B. In the above case, there is a problem that the side surfaces of the p-type GaAs contact layer 8 and the p-type second cladding layer 7, which constitute the ridge portion 10, are not covered with the n-type GaAs current constriction layer 21, and the p-type second cladding layer 7 is disadvantageously exposed similarly to the case of FIG. 5B. Although not shown, there is a similar problem concerning the ridge portion 20, the p-type second cladding layer 16 and the p-type GaAs contact layer 17 of the semiconductor laser 18.